In studies on transplant lines of 50 B lymphocytic neoplasms that arose spontaneously in old BALB/c mice and were previously established as transplant lines in this laboratory, we find that the neoplasms in many instances arise in or home to those organs where cells with the same surface immunoglobulin (Ig) isotype are most common in normal mice. Most IgM tumors homed to the spleen, a number of Ig?D tumors arose in Peyer's patches, most A tumors originated in and homed to gut associated tissue and most G tumors arose and homed to lymph nodes. The M, ?D, or A isotypes, but not the G, are stable on transplantation; that is, they are maintained through many generations of transplant. However, in some instances mice with ?D or A tumor tissue at one site had tumor tissue with either no Ig or with a different isotype at another growth site. We find evidence that tumor tissue growing outside the lymphatic tissues rather than in the spleen, nodes or Peyer's patches have lost one but not necessarily all of the surface Ig isotypes. These tumors can be of value in analyzing the stages of maturation of B lymphocytes and as experimental models of human B lymphocytes in the same stages of maturation.